The general method of operation of the type of printhead described in WO 93/11866 is well known, wherein an agglomeration or concentration of particles is achieved in the printhead, and, at the ejection location, the agglomeration of particles is then ejected on to a substrate. In the case of an array printer, plural ejection locations may be arranged in one or more rows.
Electrostatic printers of this type eject charged solid particles dispersed in a chemically inert, insulating carrier fluid by using an applied electric field to first concentrate and then eject the solid particles. Concentration occurs because the applied electric field causes electrophoresis and the charged particles move in the electric field towards the substrate until they encounter the surface of the ink. Ejection occurs when the applied electric field creates an electrophoretic force that is large enough to overcome the surface tension. The electric field is generated by creating a potential difference between the ejection location and the substrate; this is achieved by applying voltages to electrodes at and/or surrounding the ejection location.
The location from which ejection occurs is determined by the printhead geometry and the location and shape of the electrodes that create the electric field. Typically, a printhead consists of one or more protrusions from the body of the printhead and these protrusions (also known as ejection upstands) have electrodes on their surface. The polarity of the bias applied to the electrodes is the same as the polarity of the charged particle so that the direction of the electrophoretic force is away from the electrodes and towards the substrate. Further, the overall geometry of the printhead structure and the position of the electrodes are designed such that concentration and then ejection occurs at a highly localised region around the tip of the protrusions.
The ink is arranged to flow past the ejection location continuously in order to replenish the particles that have been ejected. To enable this flow the ink must be of a low viscosity, typically a few centipoises. The material that is ejected is more viscous because of the higher concentration of particles due to selective ejection of the charged particles; as a result, the technology can be used to print onto non-absorbing substrates because the material will not spread significantly upon impact.
Various printhead designs have been described in the prior art, such as those in WO 93/11866, WO 97/27058, WO 97/27056, WO 98/32609, WO 98/42515, WO 01/30576 and WO 03/101741.
In use, printheads will, at some stage, require cleaning for one or more of various reasons including removing agglomerations of ink particles from the ejection tips of the printhead or removing airborne particles from the ejection tips or intermediate electrode (IE). All previous printheads and cleaning methods were such that the cleaning was carried out by replacing all of the ink within the printhead with rinse fluid.
Such a design and process that involves replacing the ink within the printhead with rinse fluid leads to various problems. Firstly, cleaning the printhead by flushing through the ink path with rinse fluid creates a large amount of ink-rinse mixture which dilutes the ink and/or contaminates the rinse which must be filtered or discarded. It also requires the printhead to be re-primed with ink after cleaning, requiring significant time for the ink concentration to stabilise as the rinse is replaced with ink. This further causes dilution of the ink and/or mixing of a quantity of ink into the rinse, which has to be filtered out to clean the rinse.
Additionally, such a process is time consuming and, in particular when it is desired to carry out cleaning periodically to keep ejectors and intermediate electrode suitably clean to maintain good print performance for the printhead, it is desired to minimise the downtime of the printhead.
Thus the present invention is directed to reducing or avoiding entirely one or more of the problems identified above.
It has been recognised that cleaning of the ejection tips and if provided the intermediate electrode is usually sufficient to maintain print performance, and that other structures within the printhead do not require regular cleaning in operation.
According to the present invention, there is provided a method of cleaning an electrostatic printhead which has one or more ejection tips from which, in use, ink is ejected, the method comprising stopping a prior flow of ink to a region around the ejection tip(s) for, in use, printing, causing a pressure differential to occur at the tip region thereby causing the ink meniscus to retreat from the tip, and passing a rinse into the tip region to clean the tip.
Such method allows the tips to be free, or substantially free, of ink when the rinse is supplied. This ensures that the amount of ink wasted and/or rinse fluid that is required is minimised, as there are fewer regions through or across which the rinse is flowed and these regions are not filled with ink when the rinse is supplied.
One advantage of the invention is that the printhead is kept primed with ink during cleaning. Preferably, dedicated passages in the printhead direct rinse fluid and air to the tip-IE (intermediate electrode) cavity of the printhead, which is cleaned with very little mixing of rinse with ink. Ink flow around the tips is preferably stopped but the printhead remains full of ink. Air pressure in the tip region is preferably raised so that the ink meniscus retreats slightly from the tip region, exposing the tips for cleaning. Rinse may then be directed at the inside faces of the IEs from the dedicated passages within the printhead body, resulting in the cleaning of the inside face of the IEs and the tips. Rinse flow is preferably pulsed in short bursts, which helps to reduce the amount of rinse that enters the ink channels. The rinse preferably then drains into a maintenance cap sealed onto the face of the printhead during maintenance.
By using separate passages to introduce cleaning fluids to the printhead tips and IE, and by withdrawing the ink from the tips but not from the rest of the printhead, prime is maintained and cross-contamination of rinse and ink is minimised; by pulsing the flow of rinse into the printhead, alternating with air, the rinse does not flow up the ink channels significantly; by making repriming unnecessary the cleaning cycle is dramatically shortened and waste is reduced.
The ink preferably remains in the body of the printhead during the cleaning of the tips. This means that re-priming of the printhead after cleaning is therefore faster, as the ink only needs to be moved forward towards the tips rather than refilling the entire printhead. The “body of the printhead” essentially means the parts of the printhead of significant volume which would, in the normal course of operation contain ink. This includes the inlet and outlet manifolds, and typically it means that there is still ink at the base of the ink channels which connect to the respective ejection tips.
The method may further comprise the step of pulsing the flow of rinse. The pulsing may include alternating pulses of rinse and air. The pulsing may comprise pulses of air and rinse combined. The pulsing may comprise injecting rinse into an airflow. The pulsing may include air pulses, and pulses of air and rinse combined.
The air/rinse pulse is preferably 50% longer than the air pulse. The air/rinse pulse is typically 3 seconds. The air pulse is typically 2 seconds.
The printhead preferably comprises an intermediate electrode and the rinse is preferably directed at an inside face of the intermediate electrode.
The pressure differential required is preferably formed between the ink in the body of the printhead, and the atmosphere at the tip.
The pressure differential may be caused by applying a localised increase in atmospheric pressure at the tip.
The increase in atmospheric pressure at the tip may be caused by flowing air and/or rinse into the tip region.
The pressure differential may be caused by reducing the ink pressure in the body of the printhead.
The present invention also provides an electrostatic printhead comprising a main body including an inlet for ink, an array of one of more ejection tips from which in use ink can be ejected from the main body, respective channels through the main body for supplying ink to, and taking ink away from, the tips, and at least one dedicated passage extending through the main body to the ejection tips for the supply of a rinse fluid to clean the tips.
The printhead may include a datum plate having a cavity that surrounds the ejection tips, wherein the cavity is v-shaped.
The main body may also include an inflow and outflow block through which ink passes.
The angle of the “V” preferably matches a corresponding feature on the inflow and outflow block, thereby defining one or more parallel-sided fluid pathways.
A seal may be provided between the datum plate and the inflow and outflow block.
Also provided is a maintenance cap which can provide one of more of the following advantages: (i) catch and drain rinse fluid expelled from the printhead, (ii) assist in cleaning the front face of the printhead, (iii) allow the printhead to remain filled with ink during cleaning of the tips and IE, and (iv) cannot be inserted or withdrawn erroneously while clamped to the printhead.
According to the present invention, there is provided a printhead maintenance cap for attachment to a printhead, the cap comprising: a main body defining a chamber into which rinse fluid passes from the printhead during a cleaning cycle; a seal for engagement with the printhead prior to a cleaning cycle starting; and a venting system for equalising the pressure in the chamber and the surrounding atmosphere.
The printhead to which the maintenance cap is attached, in use, is may be an electrostatic printhead. The terms “maintenance cap” and “cleaning cap” are synonymous. Whilst cleaning is the preferred purpose for the cap, other tasks are also envisaged
The printhead maintenance cap may further comprise means for, in use, bringing the seal into engagement with the printhead. The engagement means includes a clamp and/or a pneumatically operated mechanism.
The venting system may include one or more baffles. The one or more baffles may be formed from a single piece component formed by stereolithography or a three-dimensional printing technique.
The printhead maintenance cap may further comprise one or more drains for draining fluid from the cap in use.
One or more additional seals may be provided to permit the cap to be used with a multi-head printhead.
The printhead maintenance cap may further comprise a movable spray head for providing one or more jets of rinse fluid within the cap.
A drive mechanism for moving the cap into and out of engagement with the printhead may be provided. This may be part of the engagement means of the printhead maintenance cap or may be separate.
The printhead maintenance cap may further comprise an interlock for preventing movement of the cap when in a sealed engagement with the printhead.
The printhead maintenance cap may further comprise a vacuum wiper. The vacuum wiper may be pivotable relative to the cap main body. The vacuum wiper may be biased towards the intended location of the printhead.
The invention also provides an electrostatic printhead having a plurality of ejection tips and an intermediate electrode, the printhead further comprising a maintenance cap as described above.
In the printhead, the vacuum wiper preferably does not contact the intermediate electrode.
Previous maintenance caps:                were not vented so draining fluid out of the maintenance cap could draw fluid out of the printhead or de-prime the printhead, necessitating prior removal of ink from the printhead.        did not seal to the intermediate electrode, but to the printhead casework which would therefore become wet internally during cleaning and necessitate a prolonged drying period.        had no protection against erroneous insertion or withdrawal of the unit while in the clamped state.        